In Our Region:
Later, around 45 million years ago, Peratherium was a conspicuous inhabitant of San Diego’s Eocene subtropical forests. Its fossil teeth and bones are particularly common in the Friars Formation. Peratherium is also known from numerous other Eocene formations in western North America.
As in modern opossums, Peratherium had a long, probably prehensile tail to hold onto branches. It also had sharp front teeth (incisors) which it probably used to capture insects. Peratherium’s four molars in the lower and upper jaw demonstrate that it was a marsupial. In contrast, placental mammals have only three molars in the lower and upper jaw.
Marsupials were plentiful during the Eocene. An even smaller opossum-like genus, Peradectes, lived alongside Peratherium in the San Diego region. Its fossils are rarer and less complete than those of Peratherium. Both of these marsupials had tiny fragile bones that are unlikely to be preserved as fossils. Usually, durable teeth are the only clues to their former presence. In rare cases such as the Eocene lake beds at Messel, Germany, whole skeletons of these ancient Eocene animals have been discovered.
By Oligocene time (about 25 million years ago), the marsupials became extinct in North America and Europe but continued to flourish in isolated Australia and South America. Much later, marsupial opossums returned to North America when the Panamanian land bridge was re-established between North and South America about 3 million years ago.
Our Virginia Opossum found foraging at dusk in local yards and gardens was introduced to the Pacific Coast in 1890.
Text: Margaret Dykens and Lynett Gillette